The present invention relates generally to protectors of the type used in central office telephone switching equipment. These protectors serve to protect the inside equipment from damage as a result of overvoltage and overcurrent conditions that may occur on the outside lines. Examples of such protectors are shown in U.S. Pats. No. 3,743,888 issued July 3, 1973; 3,573,695 issued Apr. 6, 1971; 3,587,021 issued June 2, 1971; and 3,849,750 issued Nov. 19, 1974.
Protectors of the foregoing type sometimes employ an in-line heat coil which heats a fuseable solder element. This solder element melts during an overcurrent line condition in a relatively short period of time when a marginal overcurrent condition exists. However, this requires additional heating current to be applied to the protector and, in some instances, complicates the wiring of the central office equipment. A spring within the protector is used to bring a pair of contacts together to form a direct metallic circuit from the line to ground. Under such conditions a spring is frequently relied upon to carry large overload currents of long duration. It has not always been satisfactory since the spring may not be able to carry the requisite current. In addition the heating of the spring from the overcurrent may cause it to lose its temper, resulting in separation or loose engagement with the associated pair of contacts.
Also, it is desirable that the circuit path within the protector be as short as possible and with as few as possible surface to surface contacts. Such arrangement tends to reduce the noise on the line during normal operating conditions. In the past, protectors with the heat coils have sometimes tended to be noisy due to unnecessary long current paths and multiple surface-to-surface contacts of components in the line circuit within the protector.
Another problem with line protectors of the type with which the present invention is concerned lies in the need for simplification of the insertion of the subassemblies into a protector housing of standard configuration so as to reduce the cost of manufacture of the protector. Different user requirements frequently necessitate variations in the internal structure of the protector, and, therefore, it is important of have a protector design which is versatile but which can be used on standard 5 pin base and housing structures. For example, some versions of the protector required carbon electrodes while others require the use of gas filled tubes. Moreover heat coils may or may not be required. In any event, it is preferable that protector units of the type disclosed herein be designed with a certain number of standardized or common parts so that numerous variations of the protector can be easily made.